Respiratory systems provide breathable gas, such as oxygen, anesthetic gas, and/or air directly to a patient's mouth, nose, or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit of a breathing circuit. The breathing circuit may include an expiratory limb hose or conduit to carry expelled air and other gas(es) from the patient back to the ventilator.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humidification system having a humidification chamber for holding water and a heater unit to which the humidification chamber may be releasably mounted. The humidification chamber is typically a dome-shaped plastic member with a thermally conductive metal base plate, and is typically intended to be disposable so as not to be reused from patient to patient. The heater unit includes a heater, which may be comprised of one or more heater elements and a metal plate defining a hot plate. A wall of the humidification chamber, such as the thermally conductive metal base plate, is placed into thermal contact with the hot plate of the heater, to thus heat the water in the humidification chamber. The breathable gas is coupled into and passed through the humidification chamber to be heated and humidified before being passed on to the patient, such as through the inspiratory limb of the breathing circuit. Examples of heater unit and humidification chambers are shown in U.S. Pat. Nos. 6,988,497 and 5,943,473, the disclosures of both of which are incorporated herein by reference as if fully set forth herein.
Typical heater units may also include a processor-based control responsive to one or more temperatures of the system by which to control the heater, as well as heater elements which may be associated the inspiratory and/or expiratory limbs of the breathing circuit. For example, a sensor such as a thermocouple thermally coupled to the hot plate indicates to the control the current temperature of the hot plate. By way of example, the gas temperature is monitored either at the humidification chamber outlet or at the patient. In a typical thermostatic controlled system, the hot plate heater element is energized with a fixed power level when the monitored gas temperature drops to or below a low temperature threshold so as to heat up the hot plate. The water, in turn, heats up which imparts heat and humidity to the gas passing through the humidification chamber. The heater element is then turned off when the gas temperature increases to a high temperature threshold, and the hot plate begins to cool. The water and thus the gas temperature will in turn begin to cool until the gas temperature decreases again to the low temperature threshold at which the heater element is again energized. The period of time during which the heater element is energized may be referred to herein as an “activation period” whereas the time during which the heater element is not being energized may be referred to herein as a “cooling period.” Together, an activation period and a cooling period may be seen as making up a heating cycle.
Similarly, the gas temperature to the patient can be indicated by sensors associated with the inspiratory limb adjacent the humidification chamber or adjacent the patient. One or more of those temperatures can be utilized by the processor(s) to selectively energize the hot plate heater elements and/or limb heater elements with the goal of attaining a desired temperature set point of the humidified, breathable gas. Moreover, should any of the temperatures being monitored exceed an applicable maximum level or differential, the heater unit may be shut down and/or caused to set off an alarm. An example of a processor-based control is shown in U.S. Patent Publication No. 2009/0110379, the disclosure of which is incorporated herein by reference as if fully set forth herein.
As the breathable gas passes through the humidification chamber, the water therein is depleted such as by evaporation. The humidification chamber may be manually refillable, or there may be a water source to selectively fill the humidification chamber as it empties. However, situations might arise where the water level in the humidification chamber becomes so low that the humidification chamber is effectively dry. As a consequence, the breathable gas passing through the humidification chamber exits into the breathing circuit with insufficient humidity. Current heater units are not equipped to monitor the water level in the humidification chamber, leaving it to the caregivers to devote significant time and effort to manually monitor the humidification chamber and determine if there is an acceptable level of water present. While it may be possible to automatically monitor the level with a sensor associated with the humidification chamber, that approach is not desirable. Not only is there the added cost of the sensor(s) for doing so, but the processor-based control of the heater unit would have to be adapted to communicate with an additional sensor, thus adding cost and complexity to the heater unit.